This invention relates to an optical head for recording data on and reproducing data from an optical recording medium.
Recently, together with improvements in digital signal processing technology, there have been various developments in digital recording systems. These digital systems have been used to ameliorate the problems occurring in the prior art analog system of recording, to reduce the noise and distortion in the recording signal data and to increase the recording density. For example, when using a disk coated with a metal thin film, the various data is recorded by forming pits in the disk, which correspond to the digitalized data. The recorded data is reproduced by detecting the presence or absence of the pits.
When pits are formed in this kind of disk (optical disk), the diameter of the pits should be formed less than about 1.0 .mu.m across and the track pitch should be 1.0 to 2.0 .mu.m according to increases in density. Light is used to form the pits recording the data and to detect the presence and absence of the pits for reproduction. The optical head used for recording and reproducing should have excellent tracking and focusing control functions.
In practice, when a 5 V power source is used, for example, the optical head must have a drive sensitivity such that the optical head acceleration of about 36 m/s.sup.2 in both tracking and focusing directions can be obtained with a low power consumption of 1/3 watt. In order to produce this kind of function, ordinary variable coil or variable magnet systems have been considered for the optical head drive system. There is a tendency to use the variable coil system based on prior art high acceleration one-dimensional drive system, such as the voice coils of speakers, in recent years.
With the variable coil systems, however, it is necessary to attach a lead wire to the coil for supplying power from the stationary section, resulting in deterioration of the optical head's vibration characteristics under the influence of the lead wire and making precise positioning of the head impossible. Furthermore, the terminal board required for attaching the lead wire increases the weight of the optical head, reducing the drive sensitivity.
With the variable magnet system, on the other hand, none of the above problems exist, the apparatus has superlative vibration characteristics and the assembly is simple. An example which illustrates these characteristics is the variable magnet optical head shown in Japanese Patent Laid-Open Publication 58-12145. With this apparatus, however, a force acting on a portion of the focus coil facing the magnet and force acting on the other portions of the focusing coil act in opposite directions each other, resulting in mutual cancellation of the magnetic force. Consequently, the reaction force for the magnet is extremely weak and the drive sensitivity of the optical head is insufficient. Thus, although the variable magnet system is known for its superlative vibration characteristics, it lacks practicability.